pRSS56, constructed by ligating a PvuI fragment (bp 498-2412) of pBluescript KS+ to a PvuI fragment (bp 2850-730) of pBS(+), contains the KS MCS from pBluescript KS+ and the unique NdeI and AatII sites between bla and f1 origin of pBS(+).

A fragment (1.184 kb) containing the HIS3 gene was inserted into the NdeI site and a cassette containing CEN6 and the ARS associated with histone 4 (ARSH4) was inserted into the AatII site of pRSS56. All ends were blunted.